1. Field of the Invention
This invention relates to phosphate ester base stock compositions having a novel combination of phosphate ester components and to aircraft hydraulic fluid compositions comprising such base stocks.
2. State of the Art
Hydraulic fluids used in the hydraulic systems of aircraft must meet exacting specifications set by aircraft manufacturers. Accordingly, the components of aircraft hydraulic fluids are carefully chosen to balance, among other properties, stability, compatibility, density, toxicity and the like. Whether the selected components can, in fact, be balanced to meet these specifications is unpredictable. Moreover, the amounts of individual components used in compositions which meet the specifications is not a priori predictable.
It has now been discovered that a particular combination of phosphate ester components employed in the base stock of aircraft hydraulic fluid compositions provides surprising and unexpected properties. Specifically, it has been found that by selecting particular ratios of the tri-iso-butyl and tri-n-butyl phosphate ester components of the fluid, an unexpected and surprising balance of combined properties critical to aviation hydraulic oils is obtained, including acceptable hydrolytic stability, high flash point, good anti-wear properties, acceptable erosion protection, acceptable low temperature flow properties (viscosity), and elastomer compatibility.
This invention is directed to phosphate ester base stock compositions and aircraft hydraulic fluid compositions containing a basestock having a novel combination of phosphate ester components.
Accordingly, in one of its composition aspects, the present invention is directed to an aircraft hydraulic fluid composition comprising a phosphate ester-base stock comprising a mixture of tri-iso-butyl phosphate and tri-n-butyl phosphate and a sufficient amount of one or more triaryl phosphates such that the base stock composition produces no more than 25% elastomer seal swell; an effective amount of a viscosity index improver; an effective amount of acid control additive; and an effective amount of an erosion inhibitor; wherein the amount of tri-iso-butyl phosphate ranges from about 30 to about 45 weight percent, preferably from about 30 to about 40 weight percent, based on the total weight of the fluid.
In another of its composition aspects, the present invention is directed to an aircraft hydraulic fluid composition comprising a phosphate ester base stock comprising from about 4 to about 14 weight percent, based on the total weight of the hydraulic fluid, of one or more triaryl phosphates, the remainder of the base stock comprising a mixture of tri-iso-butyl phosphate and tri-n-butyl phosphate; an effective amount of a viscosity index improver; an effective amount of acid control additive; and an effective amount of an erosion inhibitor; wherein the amount of tri-iso-butyl phosphate ranges from about 30 to about 45 weight percent, preferably from about 30 to about 40 weight percent, based on the total weight of the fluid.
In still another of its composition aspects, the present invention is directed to an aircraft hydraulic fluid composition comprising:
(a) from about 30 to about 45 weight percent, based on the total weight of the fluid, of tri-iso-butyl phosphate;
(b) from about 30 to about 45 weight percent, based on the total weight of the fluid, of tri-n-butyl phosphate;
(c) from about 10 to about 15 weight percent, based on the total weight of the fluid, of one or more triaryl phosphates;
(d) an effective amount of a viscosity index improver;
(e) an effective amount of acid control additive; and
(f) an effective amount of an erosion inhibitor.
In yet another of its composition aspects, the present invention is directed to an aircraft hydraulic fluid composition comprising:
(a) from about 30 to about 40 weight percent, based on the total weight of the fluid, of tri-iso-butyl phosphate;
(b) from about 35 to about 45 weight percent, based on the total weight of the fluid, of tri-n-butyl phosphate;
(c) from about 10 to about 15 weight percent, based on the total weight of the fluid, of one or more triaryl phosphates;
(d) an effective amount of a viscosity index improver;
(e) an effective amount of acid control additive; and
(f) an effective amount of an erosion inhibitor.
In a preferred embodiment, the above aircraft hydraulic fluids further comprise:
(g) an effective amount of a rust inhibitor or a mixture of rust inhibitors; and
(h) an effective amount of an antioxidant or a mixture of antioxidants.
In yet another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising a mixture of tri-iso-butyl phosphate and tri-n-butyl phosphate and a sufficient amount of one or more triaryl phosphates such that the base stock composition produces no more than 25% elastomer seal swell; wherein the amount of tri-iso-butyl phosphate ranges from about 35 to about 50 weight percent, preferably from about 35 to about 45 weight percent, based on the total weight of the base stock.
In another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising from about 5 to about 16 weight percent, based on the total weight of the base stock, of one or more triaryl phosphates, the remainder of the base stock comprising a mixture of tri-iso-butyl phosphate and tri-n-butyl phosphate; wherein the amount of tri-iso-butyl phosphate ranges from about 35 to about 50 weight percent, preferably from about 35 to about 45 weight percent, based on the total weight of the base stock.
In still another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising:
(a) from about 35 to about 50 weight percent, based on the total weight of the base stock, of tri-iso-butyl phosphate;
(b) from about 35 to about 50 weight percent, based on the total weight of the base stock, of tri-n-butyl phosphate; and
(c) from about 6 to about 16 weight percent, based on the total weight of the base stock, of one or more triaryl phosphates.
In yet another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising:
(a) from about 35 to about 45 weight percent, based on the total weight of the base stock, of tri-iso-butyl phosphate;
(b) from about 40 to about 50 weight percent, based on the total weight of the base stock, of tri-n-butyl phosphate; and
(c) from about 12 to about 16 weight percent, based on the total weight of the base stock, of one or more triaryl phosphates.
Preferably, the phosphate ester base stock comprises from 36 to 44 weight percent of tri-iso-butyl phosphate; from 42 to 48 weight percent of tri-n-butyl phosphate; and from 13 to 15 weight percent of one or more triaryl phosphates, based on the total weight of the base stock.